The micronucleus (MN) test, which is an indicator of genetic damage accumulated over the lifetime of cells, is one of the most suitable techniques to identify the integrated response to the complex mixture of pollutants. The aim of the study is to evaluate the genotoxic effects of thiamethoxam (TMX), a neonicotinoid pesticide widely used in agricultural activities, on rainbow trout, which is a biomarker in toxicological studies in aquatic ecosystems, by micronucleus technique. In the study, 4 groups including 10 fish in each were formed with two replicates. No substance was applied in the control group. Thiamethoxam doses of 25 and 75 mg/L were applied. As a positive control, 10 mg/L benzene was applied. In the application unit, 5000 erythrocytes were counted for each fish from the blood samples taken from the caudal vein after anesthesia and the micronucleus frequency in 1000 cells on average was determined. Liver tissue was taken after cervical dislocation and evaluated for histopathological analysis. As a result of micronucleus frequency analysis, the average micronucleated erythrocyte rate was 0.3% in the control group, 0.78% in the 25 mg/L thiamethoxam group, 1.72% in the 75 mg/L thiamethoxam group and 5.88% in the positive control group treated with 10 mg/L benzene at the 36th hour. At the end of the 6th day, MN frequency was 0.32% in the control group, 0.92% in the 25 mg/L TMX group, 2.6% in the 75 mg/L TMX group, and 7.06% in the benzene group. In histological examinations of the tissues, liver tissue in the control group was observed to have a normal histologic structure, while slight degeneration was observed in the 25 mg/L thiamethoxam group. In 75 mg/L thiamethoxam group, degeneration, necrosis areas and steatosis were observed in hepatocytes in liver sections. In the positive control group, cell infiltration and bile duct degeneration were observed in addition to hepatocyte degeneration, necrosis, and steatosis. As a result of the data obtained from the study, it was determined that the dose increase in thiamethoxam administration led to an increase in the frequency of erythrocytes with micronuclei compared to the control group. In addition, it was understood that there was an increase in the frequency and severity of lesions seen in the liver tissue with thiamethoxam dose increase. Thiamethoxam was observed to be genotoxic and cytotoxic in aquatic organisms, as well.